The present invention relates to apparatus and methods for triggering current acquisition and, more particularly, to apparatus and methods for triggering current acquisition used for motor control applications as soon as a valid state is detected on the motor drive.
Field oriented control of motor drive depends upon accurate and timely acquisition of phase currents. Additionally, the currents need to be acquired at specific conditions of the motor drive. These conditions are given in terms of pulse width modulation (PWM) signals. The phase current acquisition has to be initiated when all three PWM signals are either at logic 0 (also known as a “000 condition”) or when all three are at logic 1 (also known as a “111 condition”).
However, the pulse widths are variable among the three phases and the exact time when the acquisition needs to be started (logic 0 or logic 1) is difficult to determine. The existing implementations use center aligned PWM waves and start the acquisition when the center time is reached at either logic 0 or logic 1.
This scheme has two major drawbacks. First, the time lost by not starting the current acquisition at the instance the valid condition occurred has to be compensated by waiting for the conversion to complete when reading the current back from the analog to digital converter (ADC). Second, the overhead of synchronizing with the center time of the PWM (usually done in software) also can eat away at real time usage.
The ADCs used to actually acquire the motor phase currents need to be triggered so that they can start the signal acquisition. Typically, this is done using a software counter. In cases where a software timer (or counter) can not be coupled with generating the PWM drive and triggering the acquisition, this scheme cannot be used.
As can be seen, there is a need for apparatus and methods for starting current acquisition as soon as possible and for removing the overhead in synchronizing the center times so that the ADC can be triggered as soon as a valid state (000 or 111) is detected on the PWM drive.